All science begins in a star trek mode: go where no one has gone before and discover new things without knowing in advance what they might be. As researchers complete their initial surveys and accumulate a long list of questions, they shift to a Sherlock Holmes mode: formulate specific hypotheses and develop ways to test them. The exploration of Mars is now about to make this transition. Orbiters have made global maps of geographic features and composition, and landers have pieced together the broad outlines of the planet's geologic history. It is time to get more sophisticated.

Our team has built the Mars Science Laboratory, also known as the Curiosity rover, on the hypothesis that Mars was once a habitable planet. The rover carries an analytic laboratory to test that hypothesis and find out what happened to the early clement environment we believe the planet had. Loosely defined, a habitable environment has water, energy and carbon. Past missions have focused on the first requirement and confirmed that Mars had—and occasionally still has—liquid water [see “The Red Planet's Watery Past,” by Jim Bell; Scientific American, December 2006]. Those missions have also seen hints of geochemical gradients that would provide energy for metabolism. But none has seen carbon in a form potentially suitable for life.